The notorious long term stability, and consequent low rate of dewatering, of natural sludge in storage ponds has resulted in the accumulation of very large volumes of sludge remaining in disposal ponds at many industrial process sites around the world. Tailings ponds of the phosphate industry in Florida, of the tar sands industry in Alberta, Canada, and the bauxite industry in Jamaica are three of a multitude of examples in which this centuries old tailings pond problem persists despite the best efforts of those skilled in the art to significantly improve the sludge layer characteristics. Naturally occurring accumulations of sludges and slimes present similar problems in many areas around the world. This invention relates to means for effectively treating such tailings ponds and natural sludge and slime covered regions. The invention, while applicable to virtually all man-made and naturally occuring sludges and slimes, is discussed in the particular context of tar sands tailings ponds inasmuch as the effluent from the hot water process for extracting bitumen from tar sands is particularly difficult to deal with.
Tar sands (which are also known as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous, petroleum. Tar sands are found throughout the world, often in the same geographical areas as conventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca region in the northeast of the province of Alberta, Canada. This deposit is believed to contain perhaps 700 billion-one trillion barrels of bitumen. For comparison, 700 billion barrels is just about equal to the world-wide reserves of conventional oil, 60% of which is found in the Middle East. While much of the Athabasca deposit is not economically recoverable on a commercial scale with current technology, nonetheless, a substantial portion is situated at, or very near, the surface where it may fairly readily be mined and processed into synthetic crude oil, and this procedure is being carried out commercially on a very large scale by Great Canadian Oil Sands (now Suncor Inc.--Oil Sands Division) and Syncrude near Fort McMurray, Alberta.
Athabasca tar sands is a three-component mixture of bitumen, mineral and water. Bitumen is the valuable component for the extraction of which tar sands are mined and processed. The bitumen content is variable, averaging 12 wt% of the deposit, but ranging from zero to 18 wt%. Water typically runs 3 to 6 wt% of the mixture, and generally increases as the bitumen content decreases. The mineral content is relatively constant, ranging from 84 to 86 wt%.
While several basic extraction methods to separate the bitumen from the sand have been known for many years, the "hot water" process is the only one of present commercial significance and is employed by both GCOS and Syncrude. The hot water process for achieving primary extraction of bitumen from tar sand consists of three major process steps (a fourth step, final extraction, is used to clean up the recovered bitumen from downstream processing). In the first step, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70 to 85 wt% solids. Sodium hydroxide or other reagents are added as required to maintain pH in the range of 8.0-8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidly floats (settles upwardly) to form a coherent mass known as froth which is recovered by skimming the settling vessel. A third stream, called the middlings drag stream, may be withdrawn from the settling vessel and subjected to a third processing step, scavenging, to provide incremental recovery of suspended bitumen.
The mineral particle size and type distribution is particularly significant to the operation of hot water process and to sludge accumulation. The terms "sand," "silt," "clay" and "fines" are used in the specification as a simplified approximation of mineral paticle size wherein sand is siliceous material which will not pass 325 mesh screen, silt will pass 325 mesh, but is larger than 2 microns and clay is material smaller than 2 microns, including some siliceous material of that size. Fines includes both silt and clay, but excludes sand. It should be again noted that these designations are simplified approximations. For an elegant and in-depth discussion of particle size and type in tar sands sludges, reference may be taken to the article entitled "Mineral Particle Interaction Control of Tar Sand Sludge Stability" by Yong and Sethi which appears in The Journal of Canadian Petroleum Technology, Volume 17, Number 4 (October-December 1978).
As previously indicated, conditioning tar sands for the recovery of bitumen consists of heating the tar sands/water feed mixture to process temperature (180.degree.-200.degree. F.), physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) of the caustic or other reagents added. Under these conditions, bitumen is stripped from the individual sand grains and mixed into the pulp in the form of discreet droplets of a size on the same order as that of the sand grains. The same process conditions, it turns out, are also ideal for accomplishing deflocculation of the fines, particularly the clays, which occur naturally in the tar sand feed. Deflocculation, or dispersion, means breaking down the naturally occuring aggregates of clay particles to produce a slurry of individual particles. Thus, during conditioning, a large fraction of the clay particles become well dispersed and mixed throughout the pulp.
Those skilled in the art will therefore understand that the conditioning process, which prepares the bitumen resource for efficient recovery during the succeeding process steps, also prepares the clays to be the most difficult to deal with in the tailings disposal operation.
The second process step, called separation, is actually the bitumen recovery step since separation occurs during the conditioning step. The conditioned tar sand pulp is first screened to remove rocks and unconditionable lumps of tar sands and clay, and the reject material, "screen oversize," is discarded. The screened pulp is then further diluted with water to promote two settling processes: globules of bitumen, essentially mineral-free, float upwardly to form a coherent mass of froth on the surface of the separation cells; and, at the same time, mineral particles, particularly the sand-sized mineral, settle downwardly and are removed from the bottom of the separation cell as tailings. The medium through which these two settling processes take place is called the middlings. The middlings consists primarily of water with suspended fine material and bitumen particles.
The particle sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter which influences the settling processes most is the viscosity of the middlings, and viscosity is directly related to fines content. Characteristically, as the fines content rises above a certain threshold, which varies according to the composition of the fines, middlings viscosity rapidly reaches high values with the effect that the settling processes essentially stop. In this operating condition, the separation cell is said to be "upset". Little or no oil is recovered, and all streams exiting the cell have about the same composition as the feed. Thus, as feed fines content increases, more water must be used in the process to maintain middlings viscosity within the operable range.
The third step of the hot water process is scavenging. The feed fines content sets the process water requirement through the need to control middlings viscosity which is governed by the clay/water ratio. It is usually necessary to withdraw a drag stream of middlings to maintain the separation cell material balance, and this stream of middlings can be scavenged for recovery of incremental amounts of bitumen. Air flotation is an effective scavenging method for this middlings stream.
Final extraction or froth clean-up is typically accomplished by centrifugation. Froth from primary extraction is diluted with naphtha, and the diluted froth is then subjected to a two-stage centrifugation. This process yields an essentially pure diluted bitumen oil product. Water and mineral removed from the froth during this step constitutes an additional tailings stream which must be disposed of.
In the terminology of extractive processing, tailings is the throw-away material generated in the course of extracting the valuable material from an ore. In tar sands processing, tailings consists of the whole tar sand ore body plus net additions of process water less only the recovered bitumen product. Tar sand tailings can be subdivided into three categories; viz: (1) screen oversize, (2) sand tailings (the fraction that settles rapidly), and (3) tailings sludge (the fraction that settles slowly). Screen oversize is typically collected and handled as a separate stream.
Recently, in view of the high level of ecological consciousness in Canada, United States, and elsewhere, technical interests in tar sands operation, as well as other diverse ore handling operations, has begun to focus on tailings disposal. The concept of tar sands tailings disposal is straightforward. If one cubic foot of tar sands is mined, a one cubic foot hole is left in the ground. The ore is processed to recover the bitumen fraction, and the remainder, including both process material and the gangue, constitutes the tailings that are not valuable and are to be disposed of. In tar sands processing, the main process material is water, and the gangue is mostly sand with some silt and clay. Physically, the tailings (other than oversize) consist of a solid part (sand tailings) and a more or less fluid part (sludge). The most satisfactory place to dispose of these tailings is, of course, in the existing one cubic foot hole in the ground. It turns out, however, that the sand tailings alone from the one cubic foot of ore occupy just about one cubic foot. The amount of sludge is variable, depending on ore quality and process conditions, but averages about 0.3 cubic feet. The tailings simply will not fit back into the hole in the ground.
The historical literature covering the hot water process for the recovery of bitumen from tar sands contains little in the way of a recognition that a net accumulation of sludge would occur. Based on analysis of field test unit operations which led to the Great Canadian Oil Sands plant design near Fort McMurray, Alberta, the existence of sludge accumulation was predicted. This accumulation came to be called the "pond water problem." Observations during start-up and early commercial operations at Fort McMurray (1967-1969) were of insufficient precision to confirm the prediction. Since 1969, commercial operating data have confirmed the accumulation in GCOS' tailings disposal area of a sludge layer of fines material and water which settles and compacts only very slowly, if at all, after a few years. For a number of reasons, this sludge layer, in common with similar sludge layers observed in tailings ponds associated with mining and extracting processes of many kinds, is particularly important and difficult to deal with.
At the GCOS plant, for dike building, tailings are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constucted to serve as an impoundment for the pool of fluid contained inside. On the dike, the sand settles rapidly, and a slurry of fines, water, and minor amounts of bitumen flows into the pond interior. The settled sand is mechanically compacted to strengthen the dike as it is built to a higher level. The slurry which flows into the pond's interior commences stratification in settling over a time scale of months to years.
Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. Rapid and slow settling processes occur, but their distinction is not as sharp as in dike building, and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to form a gently sloping beach extending from the discharge position towards the pond interior. As the sand settles, fines and water commence long-term settling in the pond.
The exceedingly complex behavior and characteristics of tailings ponds only recently come to be understood beyond the simplistic categorization of various zones such as clarified water, transition, and sludge/slime. Since a tailings pond employed in conjunction with the hot water process for processing tar sands is fairly typical, the following characteristics of the layers or zones in such a tailings pond is a good general example.
Tailings from the hot water process containing a dilute suspension of fine materials in water, together with sand, are discharged to the tailings pond. The formation of sludge by settling of these tailings is attributable primarily to the presence of dispersed clay minerals. Many of the factors which determine the rate at which the clay minerals settle and the characteristics of the sludge formed are set within the tailings discharge. These include intitial clay concentration (clay/water ratio), relative proportions of various clay mineral species, particle size, condition of clay surfaces and pore water chemistry. Experience and laboratory analysis indicate that all these factors vary significantly from time to time depending on the composition of the tar sands feed and the process conditions.
Typically, tailings are discharged over the beach (either directly or from dike construction) where most of the sand settles. The run-off flows continuously into a fluid pool or pond from which water is simultaneously withdrawn as recycle to the tar sands extraction process. Here, additional important determinants of settling behavior are imposed. These include rate of inflow and outflow in relation to surface area and clarified water volume, pond depth, and degree of agitation of pond contents, either through inflows and outflows or via thermal or by wind effects. While initial temperature is inherent in the tailings streams, temperatures in the pond are obviously determined by numerous other factors as well.
Experience and laboratory analyses indicate that when a partly settled sludge remains undisturbed for between several months and about two years in a deep pond, it separates into two distinct layers, a virtually clear water layer on top and a sludge layer beneath. The density of the sludge layer increases gradually with depth due mainly to the presence of more sand and silt particles. These settle either not at all or very slowly because of the significant yield strength of stagnant sludge. The clay/water ratio increases only slightly with depth in the upper part of the pond and scarcely at all in the lower part. After one or two years, little further change in sludge volume occurs. Consolidation at the bottom of the pond is so slow that detection of consolidated material is difficult. Sludge formed in this manner is virtually unchanging over periods of years or decades and for practical purposes may be regarded as terminal sludge.
An active pond involving continuous inflow and outflow is more complex. Experience and laboratory tests indicate that, following discharge to the pond, clay particles undergo an aging process varying in length from a few days to many weeks. Prior to completion of the aging process, the clay particles do not begin to settle. However, once they commence to do so, the process proceeds quite rapidly according to the principles of Stokes Law until a clay/water ratio of about 0.13/1 is reached at which other factors evidently predominate over Stokes Law. In the uppermost part of a well managed pond, these effects result in a more or less clear water layer at the top underlaid by a layer of relatively dilute sludge more or less sharply differentiated from it. This may be termed the sedimentation zone; its volume is determined by the rate of clay inflow and the average aging time required. If the water layer is permitted to become too small in relation to the clay inflow, water outflow and aging time, the upper part of the pond becomes overloaded, the clear water layer virtually disappears and the sedimentation zone becomes much larger since clay is then recycled through the process. GCOS operated under such conditions or on the edge of them through much of the early years.
Sludge in the lower part of a deep active pond which has been in operation for some years is similar to that from an inactive pond; i.e., it may be regarded as terminal sludge. The space below the sedimentation zone and above the terminal sludge may be regarded as a transition zone lacking clear boundaries at top and bottom. It is characterized by a gradual increase in clay/water ratio with depth and owes its existence to the long time needed to attain the terminal sludge condition. Its thickness is primarily a function of the average clay inflow rate in relation to volume.
In summary, an active pond normally has a well-defined clear water layer at the top which can, however, disappear if overloading occurs. Beneath this is sludge which increases in density with depth. There are generally no clearly defined boundaries within this sludge except on occasion a layer of separated bitumen near the interface between water and sludge. However, the sludge may be considered as consisting of three zones each involving successively larger orders of magnitude of time scale for measurable dewatering to occur, and each characterized by the predominance of differing dewatering parameters. These three zones may be termed respectively a sedimentation zone, a transition zone and a terminal sludge zone.
Thus, (1) tar sands contain clay mineral, (2) in the hot water extraction process, most of the clays become dispersed in the process streams and traverse the circuit, exiting in the tailings, (3) the amount of process water input if fixed by the clay content of the feed and the need to control viscosity of the middlings stream, (4) the amount of water required for middlings viscosity control represents a large volume relative to the volume of the ore itself, and (5) upon disposal, clays settle only very, very slowly; thus, the water component of tailings is only partially available for reuse via recycle. That which cannot be recycled represents a net accumulation of tailings sludge.
The pond water problem, therefore, is to devise long-term, economically and ecologically acceptable means to eliminate, minimize, or permanently dispose of the accumulation of sludge. Experience has demonstrated that the problem requires a multifaceted approach toward its solution, and the present invention is directed at achieving one aspect of the solution: a more thoroughly dewatered sludge layer which, as a consequential result, obtains a greater quantity of clarified water for recirculation into the process if necessary in the particular system.
Flocculation of the tailings stream in order to improve the settling characteristics of an industrial process tailings pond has been proposed and practiced in the prior art. In flocculation, individual particles are united into rather loosely-bound agglomerates or flocs. The degree of flocculation is controlled by the probability of collision between the particles and their tendency toward adhesion after collision. Agitation increases the probability of collision, and adhesion tendency is increased by the addition of a flocculant.
Reagents act as flocculants through one or a combination of three general mechanisms: (1) neutralization of the electrical repulsive forces surrounding the small particles which enables the van de Waals cohesive force to hold the particles together once they have collided; (2) precipitation of voluminous flocs, such as metal hydroxides, that entrap fine particles; and (3) bridging of particles by natural or synthetic, long-chain, high-molecular weight polymers. These polyelectrolytes are believed to act by absorption (by ester formation or hydrogen bonding) of hydroxyl or amide groups on solid surfaces, each polymer chain bridging between more than one solid particle in the suspension.
A remarkable number of flocculants have been employed in the prior art to obtain precipitation of particles in tailings ponds of various industrial processes as well as in sewage treatment facilities and naturally occurring slimes. However, a distinct step forward in the art has been achieved by the use of hydrolyzed wheat, corn, and potato starch flocculants as described in U.S. Pat. No. 4,289,540 entitled "Hydrolyzed Starch-Containing Compositions" by Dr. Raymond N. Yong and Dr. Amar J. Sethi. The disclosure of U.S. Pat. No. 4,289,540 is incorporated hereinto by reference. These specific hydrolyzed starch flocculants, particularly taking into account the economics of carrying out flocculation on a large scale, enjoy high performance characteristics for their ability to bring about rapid precipitation to a substantially terminal settled condition. This characteristic is especially valuable for use in those processes, such as the hot water process for obtaining bitumen from tar sands, in which there is a critical need to recyle clarified water from the tailings pond back into the process. However, experience has indicated that the simple use of these hydrolyzed starch flocculants, or for that matter any other known flocculant, results in very little, if any, improvement on the ultimate degree of dewatering of the sludge layer. That is, the terminal status of the sludge layer is just about the same as would be obtained over a much longer period of time by natural settling processes, and this terminal condition is unsatisfactory in that it includes too much water, is too voluminous, and is too unstable.
Nonetheless, it is not accurate to say that all characteristics of a sludge layer obtained as a result of flocculation by the aforementioned hydrolyzed starch flocculants is the same as that achieved naturally or by the use of other flocculants. In point of fact, certain very desirable characteristics to the sludge layer are obtained from the use of the hydrolyzed starch flocculants which are not achieved by natural settling or by the use of any other flocculant presently known, and it is on the appreciation and use of these characteristics that the present invention is based. More particularly, it has been found that the permeability and shear strength characteristics of the sludge layer are both very much increased; as a result, previously impossible dewatering techniques may be employed to compact and stabilize the sludge layer and to extract additional amounts of clarified water therefrom.
It has been proposed in the past, as another approach to alleviating pond water problems, to store the fines in the interstices between the sand grains in the material employed for dike building. Such a process is disclosed in Canadian Pat. No. 1,063,956, issued Oct. 9, 1979, and entitled "Method of Sludge Disposal Related to the Hot Water Extraction of Tar Sands" and corresponding U.S. Pat. No. 4,008,146, issued Feb. 15, 1977. The experience with the procedure described in that reference is that the height to which the dike can be built is somewhat limited; however, it has now been discovered that if the sludge mixed with the sand to prepare the dike building material has been treated with the aforementioned hydrolyzed starch flocculants, the strength of the resultant material is notably increased such that the dike can be built higher, thereby not only permitting a deeper tailings pond, but also storing more sludge in the interstices between the sand grains comprising the dike.